Evidence that blocking is due to associative deficit: Blocking history affects the degree of subsequent associative competition.

Rats were trained on an appetitive discriminated operant procedure according to a blocking design, in which training with a compound stimulus (A + B) was given with or without prior training with A alone. Stimulus B was then presented in compound with a new stimulus (B + C compound). The ability of Stimulus B to block acquisition of control by Stimulus C was decreased by B having itself been blocked by A during the prior training. The results imply that the mechanism of blocking during the initial blocking procedure was a reduced association between Stimulus B and the reinforcer, and not a performance deficit such as that postulated by comparator models of learning.